1. Field of the Invention
The present invention relates to a wireless base station that performs spatially multiplexed communication with a plurality of mobile stations by adjusting the signals to and from a plurality of antennas to form directivity patterns during reception and transmission. In particular, the invention relates to an improvement in the formation of directivity patterns.
2. Description of the Related Art
In recent years, increasing attention has been given to mobile communication methods that use adaptive arrays to make efficient use of frequencies and improve the quality of communication. In such methods, a plurality of antennas are used, with the signal amplitude and phase being controlled separately for each antenna to adaptively control the direction in which signals propagate. A wireless base station that uses an adaptive array method controls a plurality of non-directional antennas and forms a directivity pattern where the reception sensitivity or transmission intensity is high in a direction in which a signal has been received from a mobile station when communicating with that mobile station. Here, the expression xe2x80x9cdirectivity patternxe2x80x9d refers to a representation of the intensity of signals that are transmitted or received in different directions by a group of antennas.
A wireless base station can form a directivity pattern that has high directivity in a direction in which a signal has been received from a mobile station and simultaneously has a xe2x80x9cnullxe2x80x9d directivity,toward other mobile stations. Here, xe2x80x9cnull directivityxe2x80x9d means that the pattern does not extend in a certain direction, or in other words, strong signals are not transmitted in that direction and reception is not sensitive to signals in that direction. By forming an optimal directivity pattern separately for each of a plurality of mobile stations, a wireless base station can simultaneously communicate with a plurality of mobile stations. This method is called xe2x80x9cspatial multiplexingxe2x80x9d.
FIG. 1A shows two examples of directivity patterns that are formed when a wireless base station communicates with two mobile stations using spatial multiplexing. The wireless base station forms the directivity pattern 101 with a high directivity for the mobile station a and null directivity for the mobile station b and simultaneously forms the directivity pattern 102 with a high directivity for the mobile station b and null directivity for the mobile station a.
The wireless base station changes the directivity pattern to track the movement of the mobile stations. One way of tracking such movement is as follows. The wireless base station communicates with a mobile station using time-division multiplexing where each frame is only several milliseconds (such as five milliseconds) long. When performing reception within a frame, the wireless base station adjusts the directivity pattern so that a predetermined signal located at the start of each frame is received properly. The wireless base station receives the payload of the reception signal using this adjusted directivity pattern, and then transmits a signal to the mobile station using the same directivity pattern that was used during reception within the same frame.
FIG. 1B shows the directivity patterns formed by a wireless base station, which is communicating via spatial multiplexing with the mobile stations a and b, for communication with the mobile station a. This drawing shows the respective positions of the mobile stations a and b at the times T1, T2, and T3. The times T1, T2, and T3 correspond to the consecutive frames T1, T2, and T3, so that the wireless base station forms the directivity patterns 101, 103, and 104 from the predetermined signals received from the mobile station a as part of each frame. When performing transmission within frames, the wireless base station forms the same directivity patterns 101, 103, and 104 as those used during reception in the same frames. In this way, wireless base station transmits and receives signals by forming directivity patterns in frame units for the mobile station a. When these directivity patterns are optimally formed, there is directivity toward the mobile station b is definitely xe2x80x9cnullxe2x80x9d, as shown in FIG. 1B. The wireless base station also generates directivity patterns that track the movement of the mobile station b for communication with the mobile station b. These directivity patterns are formed in the same way as the directivity patterns produced for the mobile station a.
The wireless base station forms and adjusts directivity patterns by applying weightings to the transmission and reception signals of each antenna using a set of parameters and then combining the weighted signals. A set of such parameters is called a xe2x80x9cweight vectorxe2x80x9d. The wireless base station sets a certain weight vector as an initial value. The wireless base station then receives a predetermined signal from a mobile station, such as a preamble or a synch word (or unique word), via each antenna and adds a weighting to the signal received by each antenna using the weight vector. A combined signal produced by combining these weighted signals is compared with a reference signal that is stored in advance, and the weight vector is adjusted so as to minimize the difference between the combined signal and the reference signal. By repeating this process of weighting and combining signals, comparing the combined signal with a reference signal, and adjusting the weight vector at predetermined intervals, the weight vector can be made to converge on an optimal vector around the time the payload of the signal is received from a mobile station.
The wireless base station receives the payload by using the converged weight vector to apply a weighting to each signal corresponding to the payload that is received by the antennas. During transmission, a transmission signal for the mobile station is distributed to each antenna. At this point, the transmission signal to be transmitted by each antenna is weighted using the weight vector calculated during reception in the same frame, and a weighted transmission signal is transmitted by each antenna.
In addition to using an adaptive array, the wireless base station is constructed so as to communicate using time-division multiplexing according to a TDMA/TDD (Time Division Multiple Access/Time Division Duplex) method where four channels are multiplexed together. FIG. 2 shows one example of a TDMA/TDD frame when communication is performed using both spatial multiplexing and time-division multiplexing.
The period covered by the illustrated TDMA/TDD frame is divided into eight timeslots that are numbered one to eight. The first to fourth timeslots are upstream timeslots for transmission from mobile stations to the wireless base station and the fifth to eighth timeslots are downstream timeslots for transmission from the wireless base station to mobile stations.
In the second and sixth timeslots, the wireless base station performs transmission and reception for the mobile stations a and b using spatial multiplexing. In the third and seventh timeslots, the wireless base station performs transmission and reception to the mobile station c. In the fourth and eighth timeslots, the wireless base station performs transmission and reception to the mobile station d. By performing both time-division multiplexing and spatial multiplexing, the wireless base station can simultaneously communicate with a number of mobile stations given as the number of the mobile stations that can be simultaneously handled by time-division multiplexing multiplied by the number of stations that can be handled by spatial multiplexing.
Due to problems such as weak electrical fields, interference, and losses of synchronization, there are cases where the wireless base station cannot calculate a weight vector for generating an optimal directivity pattern from the received signals. This is because the weight vector does not converge to the optimal value. An inappropriate directivity pattern is formed using this non-optimal weight vector. Such a pattern does not have directivity towards the intended mobile station or null directivity towards the other mobile stations. This problem is especially severe when the wireless base station is constructed to use the weight vector of a previous frame as the initial value for the weight vector of the present frame, as the adverse effects of earlier frames will continuously affect later frames. This prevents normal communication from being restored.
In order to stop such problems occurring when there are weak electric fields, interference, losses of synchronization, or the like, during transmission the wireless base station does not use an inappropriate weight vector that was calculated during reception. Instead, the wireless base station forms a directivity pattern using the weight vector that was calculated in the preceding frame. Since mobile stations do not move by much during the short time that is equivalent to one frame, there is a higher probability of a directivity pattern of the preceding frame being directed towards the intended mobile station that an inappropriate directivity pattern being directed in this way. This lowers the probability of the adverse effects spreading to later frames.
Weak electric fields, interference and losses of synchronization can be detected by monitoring for reception errors. Reception errors are judged to occur when a wireless base station cannot detect a predetermined synch word or when a CRC (Cyclic Redundancy Check) error is detected.
FIG. 3 shows one example of when the wireless base station detects a reception error when receiving signal from the mobile station a. As shown in FIG. 3, the wireless base station communicates using spatial multiplexing in the second and sixth timeslots in each of frames (T1), (T2), and (T3). The wireless base station is assumed to detect a reception error when receiving a signal from the mobile station a in frame (T3). The legends T1, T2, and T3 here show the times of each frame.
The following describes the operation of the wireless base station in order of the frames. In the second timeslot of frame (T1), the wireless base station receives predetermined signals from the mobile stations a and b and calculates the weight vectors Wa(T1) and Wb(T1). The wireless base station then uses these vectors when receiving the payloads of the signals received from these mobile stations a and b. In the sixth timeslot in the-frame (T1), the wireless base station uses these weight vectors Wa(T1) and Wb(T1) again to transmit respective signals to the mobile stations a and b.
In the same way, in frame (T2) the wireless base station calculates the weight vectors Wa(T2) and Wb(T2) and receives signals from the mobile stations a and b, before using the same weight vectors Wa(T2) and Wb(T2) when transmitting the transmission signals. After this, in frame (T3), the wireless base station calculates the weight vectors Wa(T3) and Wb(T3) from the predetermined signals in the signals received from the mobile stations a and b. In this example, the wireless base station detects a reception error in the signal received from the mobile station a in this frame (T3). During transmission it is inappropriate to use the weight vector Wa(T3) calculated when this reception error occurred, so that the wireless base station uses the weight vector Wa(T2) calculated for the preceding frame when transmitting to the mobile station a and the weight vector Wb(T3) when transmitting to the mobile station b.
When the same directivity pattern is formed as in the preceding frame, however, there are cases where the pattern does not have null directivity toward mobile stations that are not the intended mobile station. In this case, there is the problem of the communication between the wireless base station and the non-intended mobile station being hindered.
FIG. 4 shows a specific example of the above situation. As in FIG. 1B, the wireless base station is communicating with the mobile stations a and b using spatial multiplexing, with FIG. 4 showing the directivity patterns generated by the wireless base station for communication with the mobile station a. As in FIG. 1B, the mobile stations a and b move and are positioned as shown in FIG. 4 at the points T1, T2, and T3. These times T1, T2, and T3 correspond to the successive frames (T1), (T2) and (T3). In this example, the wireless base station forms optimal directivity patterns 101 and 103 from the predetermined signals in the signals received from the mobile station a during the frames (T1) and (T2), and forms the same directivity patterns during transmission in the same frames.
FIG. 4 differs from FIG. 1B in that the wireless base station detects that a reception error has occurred during the reception of signals from the mobile station a in frame (T3). The wireless base station is not capable of forming the directivity pattern 104 that should be formed and instead performs transmission using the same directivity pattern 103 as the preceding frame.
In this example, the mobile station b moves from the position shown for time T2 to the position shown for time T3. As a result, the mobile station b enters one of the lobes of the directivity pattern 103, so that the directivity pattern 103 has a high directivity toward both the mobile station a and the mobile station b. This means that at time T3, both the directivity pattern formed for the mobile station b (not illustrated) and the directivity pattern 103 formed for the mobile station a have a high directivity for the mobile station b, and that the signals transmitted to the mobile station b from the wireless base station are subject to interference by the directivity pattern 103. In this way, communication between the wireless base station and the mobile station b is hindered.
It is an object of the present invention to provide a wireless base station that can communicate with a mobile station for which a reception error has occurred by forming a directivity pattern that has a null directivity for other mobile stations.
The stated object can be achieved by a wireless base station that uses time-division duplexing and communicates with a plurality of mobile stations, the wireless base station repeatedly executing a cycle including a reception period and a transmission period, the reception period being where the wireless base station calculates a first weight vector for each of a plurality of mobile stations and obtains transmission signals transmitted by the plurality of mobile stations by weighting signals received by a plurality of antennas using the calculated first weight vectors for the mobile stations, the transmission period being where the wireless base station transmits signals, which have been weighted using the first weight vectors corresponding to the mobile stations, to the plurality of mobile stations via the plurality of antennas, wherein during the reception period in a cycle, if the wireless base station detects an error in a signal obtained from an arbitrary mobile station, the wireless base station calculates, from current response vectors of other mobile stations, a second weight vector that has null directivity for the other mobile stations, and during the transmission period in the same cycle as the reception period where the error is detected, the wireless base station weights a signal transmitted to the arbitrary mobile station using the second weight vector instead of using a first weight vector and transmits the weighted signal via the plurality of antennas.
As a result, the directivity pattern formed for a mobile station for which a reception error has occurred is not directed toward other mobile stations. This means that communication with this mobile station does not interfere with the directivity patterns of other mobile stations. The wireless base station can therefore communicate with the other mobile stations as normal.
The stated object can also be achieved by a wireless base station that uses a time-division duplexing and communicates with a plurality of mobile stations, the wireless base station repeatedly executing a cycle composed of a reception period where signals are received from mobile stations and a transmission period where signals are transmitted to mobile stations, the wireless base station including: a reception unit for calculating, during the reception period in a cycle, a first weight vector for each of a plurality of mobile stations, and for weighting signals received via a plurality of antennas using the calculated first weight vectors to obtain a received signal for each of the mobile stations; a transmission unit for transmitting, during the transmission period in a cycle, signals to each mobile station via the plurality of antennas, the signal transmitted to each mobile station having been weighted using the first weight vector corresponding to the mobile station; and a detection unit for detecting, during the reception period in a cycle, whether an error has occurred in a signal obtained for any of the mobile stations, wherein when the detection means detects an error in a signal obtained for an arbitrary mobile station, the transmission unit calculates a second weight vector for the arbitrary mobile station based on current response vectors for other mobile stations and uses the calculated second weight vector in place of a first weight vector to weight the signal transmitted to the arbitrary mobile station in the transmission period in a same cycle as the reception period in which the error occurred.
With the stated construction, when a reception error occurs, the directivity pattern formed by the transmission unit using the second vectors has a null directivity towards other mobile stations. This means that communication with the mobile station for which the reception error occurred does not interfere with the directivity patterns of other mobile stations, so that the wireless base station can communicate with the other mobile stations as normal.
Here, the transmission unit may include: a first calculating unit for calculating a response vector for each mobile station based on input signals inputted by each antenna and the obtained signal for each mobile station; and a second calculating unit for calculating, when the detecting unit detects that an error has occurred for an arbitrary mobile station, a second weight vector that cancels out components of the response vectors of other mobile stations that are included in each of the input signals.
Here, the transmission unit may increase transmission power when transmitting a signal that has been weighted using a second weight vector.
As a result, transmission power is raised for the directivity pattern formed using the second vectors. This increases the probability of transmission signals reaching the mobile station for which the reception error occurred, and increases the probability of synchronization being successfully reestablished between the wireless base station and the mobile station.